1. Field of the Invention
The present invention relates generally to stress and defect identification and, in particular, to a sensor for a system that employs magnetic fields to identify stress and defects in a metal piece.
2. Description of the Prior Art
Stress and structural defects in a metal piece and certain other properties of the metal piece can be identified by creating a time-varying magnetic field within the metal piece and analyzing the magnetic noise created in the metal piece by the magnetic field. In particular, one method employs a phenomenon known as the "Barkhausen effect" to identify stresses and defects in a metal piece. As is stated in column 2 of U.S. Pat. No. 3,427,872 ("the Leep et al. patent"), the Barkhausen effect is "defined in Webster's New International Dictionary (3rd Edition) as `a series of abrupt changes or jumps in the magnetization of a substance when the magnetizing field is gradually altered`". The abrupt jumps that occur in the magnetization as the intensity of the field is changed can be detected as electrical noise by a sensing coil disposed proximate or in contact with the metal piece. The noise carried by the electrical leads from the coil--commonly referred to as "Barkhausen noise"--can be fed through a suitable processing network and, if desired, to a speaker. The level of the Barkhausen noise that is generated at a location within a metal piece depends in part on the sense, magnitude and direction of the stress at that location and the microstructure of the metal. Accordingly, workers in the art have attempted to employ the Barkhausen effect and Barkhausen noise to identify stresses and defects in and some microstructural characteristics of a metal piece. A discussion of the Barkhausen effect can be found in columns 1 and 2 of the Leep et al. patent.
Most systems which employ the Barkhausen effect to identify stresses and defects in a metal piece include an energizing coil assembly and a sensing coil assembly. The energizing coil is disposed proximate the location of the metal piece under examination and is energized with a periodically time-varying signal to induce in the metal piece a periodically time-varying magnetic field. The time-varying magnetic field in the metal piece causes abrupt jumps in the magnetization of the metal piece to occur. A sensing coil is disposed near or in contact with the metal piece near the same location and detects the abrupt jumps in the magnetization and converts those jumps to Barkhausen noise on the electrical conductor of the coil. The Barkhausen noise is then fed to circuitry, usually located externally of the sensor, which can process the noise in a variety of manners, depending upon the type of information to be obtained and displayed. Ultimately, the processed Barkhausen noise is fed to a device which displays it. A system for energizing the energizing coil assembly and processing the Barkhausen noise, and a description of displays of Barkhausen noise are described in the Leep et al. patent.
Known Barkhausen sensors are suggested for use in either static or dynamic applications. Static testing involves making a relatively small number of inspections of a single piece or of each of a relatively small number of pieces. That limitation is imposed by the fact that most sensors used in static testing require physical contact between the energizing core of the energizing coil assembly and the test piece. Therefore, changing the location of inspection requires removing the sensor from the test piece and moving it into contact with the next inspection site to prevent rapid and undesirable wear of the energizing core that would result from moving the sensor from test site to test site while the core is in contact with the piece. Dynamic testing is employed in situations where the entire surface of a single metal piece must be tested or where more than one location of each of a number of test pieces must be inspected. The sensor used in dynamic testing does not require physical contact between the energizing and sensing coil assemblies and the metal piece. Therefore, there can be constant relative movement between the metal pieces under examination and the sensor.
Any system employing Barkhausen noise to locate stress and defects within a specimen depends, for proper functioning, upon the phenomenon that the level of Barkhausen noise generated in a specimen changes significantly when the sensor is moved between a location where no stress or defects exist and a location where stress or a defect is located. Therefore, variations in the level of Barkhausen noise generated in the metal piece caused by the examining process itself must be minimized to the point where proper analysis of the Barkhausen noise is not prevented. Accordingly, the effects on the generated Barkhausen noise due to magnetic self-coupling within the Barkhausen sensor, inconsistent magnetic coupling between the sensor and the metal piece and variations in the shapes of metal pieces must be minimized.
Magnetic self-coupling can occur between the energizing and sensing coils of the sensor. Often, it is necessary to examine a metal piece constructed of hard steel, in which only low levels of Barkhausen noise can be generated. The term "hard steel", as used herein, means steel having a Rockwell C hardness greater than about 50. Exemplary of such steels are (i) martensitic steels that have been either quenched or quenched and tempered; (ii) fine grained, high strength alloy steels; and (iii) carburized steels. Generally, the cores of the energizing coil assemblies of conventional Barkhausen sensors are fabricated from iron or steel. A relatively high level of magnetic noise is generated within iron and steel as the magnetic field induced in the iron or steel is varied. Accordingly, a high level of magnetic noise is generated within an iron or steel energizing core, which can be sensed by the sensing coil assembly and interferes with the low level Barkhausen noise generated in a hard steel piece.
Nonetheless, steel or iron has been used consistently as the material for energizing coil assembly cores--even for examining hard steel--for, generally, two reasons. First, the periodic signal used to energize the energizing coil assembly is of a rather low frequency. Electromagnetic devices which operate under low frequency commonly have cores constructed of iron or steel. Second, the relatively high noise generated in an iron or steel energizing core can interfere with Barkhausen noise generated in a test piece only if the energizing core and sensing coil assembly are located sufficiently proximate each other to permit electromagnetic coupling of the noise between the energizing core and sensing coil assembly. When conventional Barkhausen systems have been used to test hard steel for stress or defects, they have been used only in experimental settings and only on large pieces. Accordingly, the energizing core of the sensor has been large and, therefore, spaced a sufficient distance from the sensing coil assembly to prevent electromagnetic coupling from occurring.
However, in commercial applications, metal pieces with small cross-sectional areas must be examined. Therefore, the sensor used to examine those metal pieces must be small and the distance between the energizing core and sensing coil assembly must be reduced from that of experimental sensors. Destructive electromagnetic coupling occurs between the energizing core and sensing coil assembly, which makes it impossible to distinguish the low level Barkhausen noise generated in a hard steel piece from the high noise generated in the iron or steel energizing core. Therefore, there exists a need for a Barkhausen sensor which generates lower intrinsic noise than conventional Barkhausen sensors, and that permits distinguishing the intrinsic sensor noise from the Barkhausen signal obtained from the metal piece.
Proper magnetic coupling between the sensor and the metal piece can be achieved in several ways. A Barkhausen sensor having an energizing core that is adapted to contact the metal piece shall be referred to hereinafter as a "contact sensor". Since the magnitude of the magnetic field induced in a metal piece, and, accordingly, the level of the Barkhausen noise generated within the piece, by a contact sensor depends on the magnitude of the area of the core of the energizing coil assembly that is in contact with the metal piece, the same sensor will provide different readings for different magnitudes of contact area, making interpretation of the results difficult. Since conventional contact sensors have been employed only under experimental conditions, each sensor has been adapted for use with metal pieces of only one general shape. When used with that shape, a given constant magnitude of contact area is achieved. If the sensor were to be used in a commercial setting, the magnitude of the areas of the sensor and the metal pieces which are in contact with each other would vary with the shape of the metal piece. Thus, the magnitude of the Barkhausen noise that indicates existence of a defect would vary with the shape of the metal piece and, accordingly, analysis of the signals received from the sensor would be unduly difficult. Therefore, there exists a need for a Barkhausen contact sensor which either provides constant contact area between the metal piece and the sensor, regardless of the shape of the metal piece, or electrically maintains a uniformly varying magnetic field among metal pieces of various shapes.
A Barkhausen sensor having an energizing core that is not adapted to contact the metal piece under examination shall be referred to hereinafter as "air gap sensor". Use of an air gap sensor requires the existence of an air gap between the energizing and sensing coil assemblies and the metal piece. Generation by the sensor of intelligible information, in the form of Barkhausen noise, depends on the maintenance of an air gap of uniform depth throughout the examination area defined by the sensor and upon maintenance of an air gap of a constant depth from inspection point to inspection point either within the same metal piece or among metal pieces. If dynamic or continuous examination of a metal piece or pieces is desired, an air gap of a constant depth must be maintained during the period of relative movement between the metal pieces under examination and the sensor. Accordingly, there is a need for a Barkhausen sensor that provides an air gap of a consistent configuration during performance of static or dynamic examination.
Finally, the thickness of the metal piece itself can change the level of Barkhausen noise generated within the metal piece. Accordingly, examination of metal pieces of different thicknesses can require constant calibration of the system that analyzes the Barkhausen noise and makes it available for inspection. Because interpreting Barkhausen noise differently for different thicknesses of metal pieces is unduly inconvenient, it is common to normalize the system each time the thickness of the test piece changes by altering the magnitude of the current supplied to the energizing coil assembly. The proper current level can be determined in advance experimentally for different thicknesses of metal pieces.
Known Barkhausen sensors have not successfully addressed the concerns described above. Because of its low intrinsic Barkhausen noise level, it has been proposed in Finland Patent No. 51873 to use ferrite as the core material for a Barkhausen noise sensor adapted to investigate flat metal pieces; however, in that proposal, a single coil serves as both the energizing coil and the sensing coil. The result of that arrangement is that the sensing coil tends to detect the intrinsic noise of the core material rather than the noise induced in the specimen. This is especially the case when measuring hard steels in which only low levels of Barkhausen noise can be generated. Accordingly, that arrangement cannot be successfully used to detect Barkhausen noise in hard steels due to magnetic self-coupling occurring within the sensor.
An article entitled "Measurement of Barkhausen Noise On Moving Steel Sheet with Non-Contact Sensor", published in Finland by Markku Pesonen, discloses a concept for dynamically measuring Barkhausen noise generated in a steel sheet. FIGS. 5.3(a) and (b) of the publication illustrate the sensors in diagrammatic form. FIG. 5.3(a) shows a DC sensor having energizing coils energized by a DC source and FIG. 5.3(b) shows an AC sensor having energizing coils energized from an AC source. Both sensors employ identical sets of energizing and sensing coils on both sides of a steel sheet to reduce the effect of vibration of the sheet. The DC sensor employs two sets of energizing coils, each set of which is energized by a DC source of a polarity opposite to that which energizes the remaining set. The DC sensor relies on movement of the sheet relative to the coils to generate a time varying magnetic field, which causes Barkhausen noise to be generated, in the sheet.
Several problems are experienced through use of the DC sensor shown in FIG. 5.3(a). First, the DC sensor can be used to test only relatively soft metal pieces if the piece must be moved at a slow speed. At slow speeds, insufficient Barkhausen noise is generated within hard metal pieces. Examples of metal pieces which must be moved at a low rate of speed include odd shaped pieces which simply cannot be moved rapidly. Further, variations in the speed of movement of the metal sheet under observation change the "frequency" of the apparent time-varying field and introduces substantial variation in the generated Barkhausen noise that is not due to the presence or absence of stress or defects. Finally, the AC and DC sensors shown in FIGS. 5.3(a) and (b) are suitable only for inspection of flat, relatively thin metal sheets, which is a serious limitation on their use. Accordingly, the DC and AC sensors do not provide Barkhausen sensors that are practical for use during dynamic testing.
Accordingly, there exists a need for a sensor that minimizes magnetic coupling within the sensor. Further, there is a need for a sensor that provides proper coupling between the sensor and metal pieces of differing shapes and thicknesses.